Method of blasting and ammonium nitrate explosive composition



United States Patent Ofilice 3,094,069 Patented June 18, 1963 3,094,069 METHOD OF BLASTING AND AMMONIUM NITRATE EXPLOSIVE COMPOSITION Joseph R. Hradel, Mount Pleasant, and Carl Kenneth B orlr, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Jan. 5, 1959, Ser. No. 784,894

5 Claims. (Cl. 102-23) This invention relates to an improved ammonium trate explosive composition and more particularly concerns a cap-insensitive ammonium nitrate explosive composition oontaining particulated metal.

It has been a desideratum in the explosives art to formulate and compound explosive compositions which may be safely handled and transported, yet reliably and effectively detonated. To fill this need various ammonium nitrate explosive compositions have been proposed but these have been general] cap-sensitive and have required boosters, such as gelatine dynamite, spaced throughout a large load to ensure good propagation.

It is an object of the present invention to provide a cap-insensitive ammonium nitrate explosive composition which may be safely handled and transported with little danger of shock initiation yet may be detonated effectively by a shaped charge using the Munroe jet principle.

It is another object of the invention to provide a capinsensitive ammonium nitrate explosive composition which upon proper initiation produces a detonation of useful magnitude and force.

An additional object of the invention is to provide a cap-insensitive ammonium nitrate explosive composition containing a particulate light metal.

A still further object of the invention is to provide a low-cost cap-insensitive ammonium nitrate explosive composition which is adapted to be readily compounded near the point of use.

Other objects and advantages of the invention will become apparent to one skilled in the art upon becoming familiar with the following description and claims.

These and other objects are attained upon admixing ammonium nitrate, itself cap-insensitive, with a particulate light metal and with a petrolic liquid in proportions hereinafter more fully described and further illustrated by examples of the invention whereby an admixture is formed having both liquid and solid phases. All proportions herein given in percent are to be understood as percent by weight unless otherwise indicated.

The composition of the invention may be compounded with fertilizer grade as well as with explosive grade ammonium nitrate. While the explosive grade material tends to detonate with slightly greater force on an equal weight basis, it is generally more economical to make up for this by using a slightly greater amount of fertilizer grade ammonium nitrate.

Fertilizer grade ammonium nitrate generally contains various additives or fine particulate coatings inhibiting caking and promoting free-flowing characteristics of the material. Ammonium nitrate as referred to herein is defined as ammonium nitrate containing up to 3 percent of various additives such as wax, diatomaceous earth and chalk in addition to a moisture content ranging up to about 1% percent. Either the granular or pn'lled form of the nitrate salt are satisfactory.

To avoid making cap-sensitive mixtures the composition is best prepared from ammonium nitrate having particle sizes mainly in the range of 8 to 100 mesh, fines passing 100 to 200 mesh tending to increase the sensitivity of the mixture. An example of a satisfactory granular ammonium nitrate readily obtained commercially has the following sieve analysis:

TABLE I Number of Number of Percent sieve I passed sieve retained retained 200 Pan 4. 4

! Sieves of the U.S. Sieve Series, U.S. Bureau of Standards.

An example of a satisfactory prilled ammonium nitrate which is commercially available has the following sieve analysis:

TABLE 11 Number of sieve passed Number of Percent sieve retained retained The above examples are not intended to be limitative but indicative of the nature of commercially available ammonium nitrate.

The preferred light metal in particulated form is selected from the class consisting of magnesium and its alloys, aluminum and its all ys. and mixtures thereof, theoretical considerations indicating both magnesium and aluminum in reacting with ammonium nitrate evolve a greater amount of energy than most other common metals on an equal weight basis. Suitable metals and alloys which may be used in particulate form include magnesium and the magnesium alloy having the ASTM designation ZK60, magnesium alloys having the compositions indicated by the ASTM-YP designations ZKlO and A241, a magnesium alloy having a composition corresponding to the ASTM-type designation AZll with 1 percent of manganese, ZK6O alloyed which additionally contains 2 percent of thorium, magnesium alloyed with 2.8 percent of aluminum and 8.4 percent of Zinc, magnesium-aloudnum alloy containing 33 percent of aluminum, aluminummagnesium alloy containing 30 percent of magnesium in addition to aluminum metal. Particulate magnesium, and magnesium base alloys containing aluminum only or minor amounts of other alloying elements are to be preferred in preparing explosive mixtures in the form of a slurry since the densities of magnesium and solid ammonium nitrate are nearly equal. The tendency for segregation of metal particles upon settling of solids from a composition of the invention is thus very slight.

While the particulate metals may be used in the form of ground or flaked particles a desirable form is that of atomized pellets, for example atomized metal pellets ob tained as described in U.S.Patents 2,699,576 and 2,728,- 107.

Atomized metal pellets produced according to these U.S. patents referred. to above are readily formed in a narrow range of particle Sizes little lines. As to detonability, the same qif metal particle sizes generally apply whether the metal is ground or pelletized. However, slurries'contaiuing more nearly spherical atomized metal pellets tend to be pumpable at lower liquid levels than sluru es containing angular irregularlyshaped particulate metal. The following is a. sieve analysis of a suitable pelletized magnesium base alloy having the ASTM designation ZK60:

TABLE HI Sieve passed Sieve retained Percent on retained 20 35 22 35 48 38 48 65 27 65 100 100 Pan 3 Ground metals, as produced, are generally quite varied in particle size and usually contain a substantial amount of fines passing 200 and 325 mesh sieves. Not only are these fines rather easily ignited in air but they contribute to low level initiation of pa 'culated ammonium nitrate in admixture therewith. Such fines are thus unsuitable in the preparation of cap-insensitive ammonium nitrate explosive mixtures. Ground metals may be used upon separating the fines, as by screening, and selecting particles at least 85 percent of which are retained on a 200 mesh sieve and not more than 1 percent of which pass a 325 mesh sieve. It is also desirable to reject coarser metal particles not passing a 20 mesh sieve as these are too large to react effectively during the brief interval of the detonation reaction of ammonium nitrate, though the presence of a small percent of larger metal particles does not particularly adversely aifect detonability of an ammom'um nitrate. For ease of mixing and handling or pumping, the composition of the invention it is to be preferred that the particulate solid light metal used have a particle size range, mainly, of 28 to 100 mesh, not more than 1 percent of the metal passing a 325 mesh sieve.

Suitable petrolic liquids are crude oil and liquid hydrocarbons derived or fractionated therefrom, these liquids having little or no solvent action on ammonium nitrate. Specific examples of petrolic liquids successfully tested as components of the invention include crude oil, fuel oil, lubricating oil fractions and mixtures thereof, gasoline and kerosene. Petrolic liquids of low volatility such as oils are to be preferred.

The hereinabove described components may be comblood in various proportions, according to the invention, the proportions ranging from about 50 to 96 percent of particulate ammonium nitrate, from 2 to 65 percent of a solid particulate light metal and from about 0.5 to 20 percent of a petrolic liquid. A more preferred range of proportions from the standpoint of economics as well as work potential is from about 70 to 95 percent of particulate ammonium nitrate, 2.5 to 35 percent of a solid particulate light metal and 3 to 10 percent of a petrolic liquid.

While the of the invention are not readily pumpahle they are generally readily loaded directly into accessible boreholes. If desired the mixtures may be first placed in suitable deformable containers such as plastic bags and the containers placed in juxtaposition in the borehole.

The compositions of the invention possess the advantags that they may also be slurried in an additional proportion of petrolic liquid and pumped or otherwise transferred as a non-detonaible slurry into a borehole in an earth formation where the solids may be allowed to settle to form a detonable composition. Mixtures of particulate ammonium nitrate, a solid particulate light metal and a petrolic liquid in which the proportion of petrolic liquid is about 20 percent or more and preferably at least 30 percent, are generally pumpable as a slurry.

In tests to determine the relative proportions of solid and liquid phase materials settling from a pumpable slurry 100 gram portions, respectively, of granular and prilled ammonium nitrate were slurried in 100 gram quantities of a medium viscosity lubricating oil. After a half-hour period, during which the solids were allowed to settle, the mpernatant oil was decanted and measured.

The proportion of oil retained with the settled solids varied from about 6 to 18 percent indicating that upon allowing a slurry of the composition of the invention to settle detonable proportions are formed in a relatively short time.

In detonating the explosive mitxure of the invention a load or charge of the said mixture is placed in the desired location, usually in the confinement of a borehole. A shaped charge such as a jet perforator containing a charge of RDX is placed next to the load. This shaped charge in turn is armed with a suitable initiator such as a Number 8 electric blasting cap. The lead from the blasting cap is run to a remote control switch and, if desired, a sand or gravel tamp is placed over the load and initiator. The load is then fired from the remote control switch. In detonating a large load, such as one loaded into an elongated borehole, only one shaped charge is needed to detonate the entire load, though more than one may be used if desired.

To demonstrate the properties of the ammonium nitrate explosive mixture of the invention various embodiments of the invention were prepared and tested. In each test a 10 to 25 pound quantity of explosive mixture was formulated of a particulate fertilizer grade of ammonium nitrate, a solid particulate light metal and a petrolic liquid in the proportions and amounts shown hereinbelow in Table IV. The individual proportions were in each case mixed in a separate polyethylene plastic bag of suflicient size to readily hold the quantity prepared. The various com ponents of the mixture were weighed into the bag, the bag closed and the contents mixed together by kneading the bag with the hands. The fertilizer grade of ammonium nitrate contained about 0.7 percent of wax, 1 percent of diatomaceous earth and 0.3 percent of chalk. The particle size of the amomnium nitrate was such that 94 percent by weight of the particles passed a 20 mesh sieve and percent by weight were retained on a mesh sieve. Flake aluminum used was a coarse grade which had been sieved so as to select the particulate metal passing a 40 mesh sieve but retained on an 80 mesh sieve. The atomized aluminum employed was of a particle size such that 0.4 percent passed a 40 mesh sieve, about 85 percent was retained on a mesh sieve and about 0.9 percent passed a 200 mesh sieve.

The prepared mixtures were loaded into individual shallow boreholes drilled into clay soil and having a diameter of 4 inches and a depth of about 4 feet. Successive boreholes were spaced about 20 feet apart. The loading of each hole was accomplished by first placing an initiator in the form of a shaped charge armed with a blasting cap at the bottom of the hole and running the lead wires of the cap to a firing control switch. Each shaped charge used was positioned with the jet end or firing axis facing upwardly. The explosive mixtures contained in the bags in which they were mixed were dropped into respective test holes, the bags deforming so that the mixture covered the initiator. Sand was used as a tamp, the hole being filled from the bag to ground level with sand. Detonation of the mixture was attempted by closing the firing switch, thus setting off the initiator at the bottom of the hole. The magnitudes of the detonations obtained were determined by measuring the size of the crater produced. While the crater size alone is not indicative of the amount of earth formation that is broken up, it does give an indication of the work potential of the mixture detonated. The crater size herein reported shows how much material was thrown sufiiciently so as not to fall back over the test hole. Test conditions and results are summarized in Table IV.

The results in Table IV show that a broad range of compositions within the scope of the invention are detonable upon initiation with a shaped charge.

In order to compare the composition of the invention with conventional explosive varied amounts of 60% dynamite each with a No. 8 electric blasting cap therefor TABLE IV Sensitizer Petrolic liquid Weight of Crater dimensions,

Weight of explosive it. Test No. charge, RDX in lbs. shaped Percent Metal Particle typo Psr- Type Percharge, 01.. Diameter Depth FGAN cent cent 1 87 Al atomized 10 3 25 l 10 4 2. 89. 5 Al do 10 0.5 25 1 9 3. 5 3 87 10 3 25 l 3 1 4. 88 10 2 10 l 5 2 5 89. 5 l .5 l0 1 2 a. s9 1 10 1 9 2. 5 7. 89. 5 10 5 l0 1 11 4 8. 95 2. 2. 5 10 1 12 8 9, 60 25 15 10 l 10 8 10 65 30 5 l0 1/ l2 10 11 55 40 5 1o 19 16 10 i2 40 55 5 10 l 6 4 13 25 65 10 10 i 4 2 1 a 2 a 3 i 1 l l 16 7s s in 19 12 9 17 as 10 s 10 19 14 .6 1s 50 36 15 1o 1% e 1 N o'rn.FGAN=iertilizer grade ammonium nitrate. were placed in shallow testholes 4 feet deep, tamped with sand and detonated. 5 lbs. 'of dynamite so loaded and detonated produced a crater 5 feet in diameter and 1.5 feet deep; 10 lbs. of dynamite produced a crater 8 feet in diameter and 1 foot deep; while lbs. of dynamite produced a crater 11 feet in diameter and 5 feet deep.

In an additional test series a number of samples of atomized metals were compared as sensitizers in a composition of the invention. The metals tested included magnesium, magnesium alloys and aluminum and aluminum alloys. In each test 10 grams of atomized metal pellets was admixed mechanically with 90 grams of fertilizer grade ammonium nitrate in a 4 ounce narrow mouth glass bottle and 10 cubic centimeters of crude oil were then added to the admixture. The bottles were left open at the top and detonation was attempted eiher by placing a No. 8 electric blasting cap directly on the mixture or a shaped charge armed with a No. 8 electric blasting cap was placed on the open bottle with the jet end or firing axis facing downwardly. Detonation of the samples was attempted in an open crater, the samples being positioned each in turn about 8 feet from a recording barograph which was used to evaluate the relative maximum pressure produced on detonation of the various mixtures. A shaped charge alone and an unsensitized ammonium nitrate sample were used as blanks for these tests. The results recorded by the barograph in terms of scale units are listed in Table V, in addition to the metals and alloys tested.

TABLE V Comparison of Sensz'rizers Employed With NH NO Petrol|c Liquid Mzxture Maximum pres- It-st Sensitizer, metal or alloy Initiator sure ot detonation No. in barographic recorder units 19.--" ZKiO Shaped 9 avg. of 5 tests.

charge 21L ZKlO, spheres -d 10.6 avg. of 5 tests. 21 ZKlO, irregular spheres 10.7 avg. of 5 tests 22. Cell M 1 13.1 avg. of 5 tests. 23. i 10.7. 24 ZK60 9.3. 25 A241 11.4 26 AZ11+Mn 1% 9.5 27- ZK60 98% Th 2% 14.3 28. AIL 12.8%, Zn 8.4%, ha]. 10.4

g. 29. A1 33%, Mg 67% 10.3. 30. A1 70%, Mg 30% do 11.3. 31 ZKID Nil. cap only. 52... Shaped charge only .d 5.6. 53"... NH41N0; shaped charge do 6.5.

on y.

l Pressure of Detonation Shock Wave. 2 From commercial electrolytic cells.

S.F. oil=a medium viscosity semi-refined oil.

The pantieulated metals and alloys tested were screened to remove all particles except those in the range of 28 to 100 mesh (U.S. Standard Sieve Series). Each shaped charge employed contained an explosive load weighing 1% ounces of RDX.

The results of tests 19 through 30 show that magnesium and aluminum and various magnesium and aluminum base alloys are suitable sensitizers for the composition of the invention. Test 31 demonstrates the low level of sensitivity of the present explosive composition. Tests 32 and 33 run as comparative blanks show that the fertilizer grade of ammonium nitrate tested is detonahle, when not sensitized, with but little greater force than the shaped charge itself, in contrast to the results of detonatlng the mixtures of the invention.

In a test to demonstrate the detonability of the mixture of the invention after pumping as a slurry in additional petrolic liquid, a perforating gun loaded with 28 shaped charges and armed with a time bomb was lowered to the bottom of a 5,267 ft. borehole. Each shaped charge contained 1 oz. of the explosive RDX. Crude oil was then pumped into the hole to establish the strata fracturing pressure. 300 lbs. of atomized pellets of ZKlO magnesium alloy and 2700 lbs. of 35 to mesh fertilizer grade ammonium nitrate were slurried in 13 barrels of a medium viscosity semi-refined oil by means of a paddle mixer, the slurry then being pumped into the borehole at the strata fracturing pressure. A second slurry of 500 lbs. of fine and 200 lbs. of coarse fertilizer grade ammonium nitrate were similarly injected. A spacer of approximately 12 barrels of the medium viscosity semirefined oil then was pumped into the borehole followed by a sand-oil slurry tamp calculated to fill 220 ft. of Well casing. All slurries were pumped at the rate of 2 /2 barrels per minute. Crude oil was then pumped at the rate of 5 barrels per minute to fill the remainder of the casing. The well was scaled and the solids were allowed to settle over night. Detonation was initiated the next day by the time bomb. Seismic measurements at ground level indicated the entire load of ammonium nitrate explosive mixture was detonated.

What is claimed is:

l. The improved method of blasting which comprises preparing an admixture comprising of from about 60 to percent by weight of solid particulate ammonium nitrate having a particle size of from about 8 to mesh, from 2.5 to 35 percent of a solid particulate light metal selected from the group consisting of magnesium, magnesium-base alloys, aluminum, aluminum-base alloys and magnesium-aluminum binary alloys, said particulate light metal having a particle size of from about 28 to 100 mesh, not more than 1 percent of the said metal particles passing a 325 mesh sieve, and from about 3 to 10 percent of a petrolic liquid selected from the group consisting of crude oil and liquid crude oil fractions, slurrying said admixture with an excess of said petrolic liquid, pumping said slurry into a borehole in an earth formation, settling said admixture whereby a detonable composition is formed in the bottom of said horehole and detonating the said admixture with a detonator having initiating power at least as great as a 1 ounce charge of cyclotrimethylenetrinitramine.

2. The improved method of blasting which comprises admixing from about 44 to 78 percent by weight of solid particulate ammonium nitrate having a particle size of from about 8 to 100 mesh, from 2 to 30 percent of a solid particulate light metal selected from the group consisting of magnesium, magnesium-base alloys, aluminum, aluminum-base alloys and magnesium-aluminum binary alloys, said particulate light metal having a particle size of from about 28 to 100 mesh, not more than 1 percent of the said metal particles passing a 325 mesh sieve, and from about 20 to 30 percent of a petrolic liquid selected from the group consisting of crude oil, liquid crude oil fractions, pumping the so-prepared admixture into a borehole in an earth formation, allowing the admixture to settle whereby a detonable composition is formed in the bottom of the borehole, and detonating said detonable settled composition.

3. A blasting-cap insensitive explosive composition consisting essentially of from about 50 to 95 percent by weight of solid particulate ammonium nitrate having a particle size of firom about 8 to 100 mesh, from 2 to 65 percent of a particulate light metal selected from the group consisting of magnesium, magnesium base alloys, aluminum, aluminurrvbase alloys, and magnesiumeluminum binary alloys, said particulate light metal having a particle size of from about 20 to 200 mesh, not more than 1 percent of the said metal particles passing a 325 mesh sieve and from about 0.5 to 20 percent of a petrolic liquid selected from the group consisting of crude oil and liquid crude oil fractions.

4. The composition as in claim 3 in which up to 100 percent of the said solid particulate ammonium nitrate is of a fertilizer grade.

5. A blasting-cap insensitive explosive composition consisting essentially of from about to percent by weight of solid particulate ammonium nitrate having a particle size of from about 8 to mesh, from 2.5 to 35 percent of a solid particulate light metal selected from the group consisting of magnesium, magnesium-base alloys, aluminum, aluminum-base alloys and magnesiumaluminum binary alloys, said particulate light metal having a particle size of from about 28 to 100 mesh, not more than 1 percent of the said metal particles passing a 325 mesh sieve and from about 3 to 10 percent of a petrolic liquid selected from the group consisting of crude oil and liquid crude oil fractions.

References Cited in the file of this patent UNITED STATES PATENTS 1,301,646 Burrows et a1. Apr. 22, 1919 1,992,217 Kirst et a1. Feb. 26, 1935 2,398,071 Barab Apr. 9, 1946 2,703,528 Lee et a1. Mar. 8, 1955 2,867,172 Hradel Jan. 6, 1959 2,903,969 Kol be Sept. 15, 1959 FOREIGN PATENTS 486,158 Canada Sept. 2, 1952 797,112 Great Britain June 25, 1958 OTHER REFERENCES Cooley Missouri School of Mines and Metallurgy, Bullctin. Technical Series No. 95, November 14 and 15, 1957, pages -127. 

1. THE IMPROVED METHOD OF BLASTING WHICH COMPRISES PREPARING AN ADMIXTURE COMPRISING OF FROM ABOUT 60 TO 95 PERCENT BY WEIGHT OF SOLID PARTICULATE AMMONIUM NITRATE HAVING A PARTICLE SIZE OF FORM ABOUT 8 TO 100 MESH, FROM 2.5 TO 35 PERCENT OF A SOLID PARTICULATE LIGHT METAL SELECTED FROM THE GROUP CONSISTINNG OF MAGNESIUM, MAGNESIUM-BASE ALLOYS, ALUMINUM, ALUMINUM-BASE ALLOYS AND MAGNESIUM-ALUMINUM BINARY ALLOYS, SAID PARTICULATE LIGHT METAL HAVING A PARTICLE SIZE OF FROM ABOUT 28 TO 100 MESH NOT MORE THAN 1 PERCENT OF THE SAID METAL PARTICLES PASSING A 325 MESH SIEVE, AND FROM ABOUT 3 TO 10 PERCENT OF A PETROLIC LIQUID SELECTED FROM THE GROUP CONSISTING OF CRUDE OIL AND LIQUID CRUDE OIL FRACTIONS, SLURRYING SAID ADMIXTURE WITH AN EXCESS OF SAID PETROLIC LIQUID, PUMPING SAID SLURRY INTO A BOREHOLE IN AN EARTH FORMATION, SETTLING SAID ADMIXTURE WHEREBY A DETONABLE COMPOSITION IS FORMED IN THE BOTTOM OF SAID HOREHOLE AND DETONATING THE SAID ADMIXTURE WITH A DETONATOR HAVING INITIATING POWER AT LEAST AS GREAT AS A 1 OUNCE CHARGE OF CYCLOTRIMETHYLENETRINITRAMINE. 